Usually a vacuum leak of some sort. Hoses tend to be more pliable when warm and a split or perished hose may leak under only those specific conditions. Check for the common PCV hose failure problem and all the rubberr fittings that attach to the vacuum take-offs on the back of the intake manifold (just below the throttle body between the manifold and firewall.)
Vacuum leaks don't usually throw codes unless they affect the AF mixture enough to cause a lean running condition.